Add BOLT 12 invoice to payment flow

[sbddesign]

Currently, our payment flow supports BOLT 11 only. We should support BOLT 12 here. I think this is where it makes sense to actually use the BOLT 12 invoice lni... instead of an offer lno.... For any kind of order fulfillment system with bitcoin payments, you need to be able to distinguish one bitcoin payment from another. If we have tons of 5k sat payments all coming in to the same offer, we don't have a great way to correlate a payment with a given order. Thus, I think the BOLT 12 invoice would be a good solution here. But I'd love to hear others thoughts.

[sbddesign]

Notes from call:

• Support paying to BOLT 11 and BOLT 12
• Default to BOLT 11 for older wallets?
• Shiny button to draw attention to the BOLT 12 invoice (mostly devs will be interested in this)